Interest in flywheels as energy storage devices has increased recently as a result of the movement toward “green” energy produced from windmills, as such flywheels could be used in a windmill-based electrical power station to provide power output during periods when ambient winds are low. Such flywheels may also be used in solar-based electrical power stations to provide power output after sunset.
Flywheel energy storage has a number of advantages that make it an attractive design option. Compared with other ways to store electricity, flywheel energy storage systems have long lifetimes, lasting decades with little or no maintenance. Full-cycle lifetimes quoted for flywheels range from between 105 and 107 start-stop cycles of use. Such systems also have a potentially high energy density (100-130 W·h/kg, or 360-500 kJ/kg), in addition to large maximum power output. The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%. Typical capacities range from 3 kWh to 133 kWh. Rapid charging or discharging of a flywheel system can occur in less than 15 minutes.
Because the energy storage capacity of a flywheel increases with the square of its rotational speed, most energy-storing flywheel systems are designed to operate at very high rotational speeds (e.g. 10,000-60,000 rpms or more). But while such high speed rotation advantageously increases the energy storage capacity of the system, it also results in a number of disadvantages. High speed flywheels must be precision-constructed of high tensile strength material to maintain balance and structural integrity during operation. To cope with the frictional losses associated with such high speeds, precision bearings are necessary. In some systems, conventional ball or roller bearings are used wherein the ball or roller bearings are caged within concentric races. To reduce friction to acceptable levels and to insure longevity, the cages, races and balls or rollers of such mechanical bearings must be machined to exacting tolerances. To further reduce friction, some flywheel energy storage systems use pressurized air or repulsive magnet bearings. However, such levitating-type bearings substantially increase the costs associated with such systems and impose practical limitations on the weight of the flywheel. Because of the potential dangers associated with an explosive shattering of the flywheel caused by the large centrifugal forces generated at such high speeds, it is necessary to contain them within a thick-walled housing or a recess in the ground. For flywheels rotated at speeds high enough to exceed the sound barrier around their periphery, it is desirable to at least partially evacuate the interior of the housing to eliminate the resulting turbulent air drag losses, and the wear around the outer edge of the flywheel due to air friction. All of these requirements increase the overall cost of the system.